The physical properties of rubber vulcanizates have traditionally been modified with particulate fillers to change their stiffness and hardness for specific applications such as tires. Conventional fillers include carbon blacks, precipitated silicas, clays, and talc. Carbon black has been highly preferred for tire applications as it contributes not only to the stiffness of the cured vulcanizate but also imparts added strength and toughness. The availability of a multitude of carbon blacks with different surface areas, particle sizes and structure has allowed researchers to quantify the variables that determine rubber reinforcement. Particulate fillers with high surface areas per gram of material, good surface interaction with rubber, and high structure contribute more to rubber reinforcement than particulate fillers with lower amounts of these three variables. Carbon black tends to have better surface interaction with rubber than does silica making it a preferred filler. The manufacturing technologies for carbon black and silica allow easy control over the surface area per gram and the structure of both types of fillers.
Silicas include precipitated silica, fumed silica, natural deposits high in SiO.sub.2 and manufacturing byproducts high is SiO.sub.2. Fumed silica is highest in price and more often is used in silicone rubbers than diene based elastomers. This is predominantly due to its higher cost per pound. Precipitated silicas are used in significant quantities in diene based elastomers.
Precipitated silicas are prepared by neutralizing a soluble silicate such as sodium silicate under controlled conditions such as temperature, silicate concentration, pH, electrolyte concentration, and controlled particle nucleation during particle growth cycles. Neutralizing agents for sodium silicate include hydrochloric acid, sulfuric acid, phosphoric acid, and carbonic acid. After a precipitated silica of the desired characteristics is formed the silica is generally separated from the excess water by filtration and washed to remove most of the nonsilica byproducts, e.g. Na.sub.2 SO.sub.4, NaCl, NaHCO.sub.3, Na.sub.2 HPO.sub.4 and NaH.sub.2 PO.sub.4 or Na.sub.2 CO.sub.3. Inorganic salts are not conventionally added to rubber compositions and silica manufacturers expend effort to reduce the inorganic salt content to less than 0.5 wt. %. Besides certifying particle size surface area, and pH most silica manufacturers certify less than 0.5 wt. % inorganic salt. The silica is generally dried to less than 10 wt. % total water and the particle size adjusted by milling and/or air classification as desired. For increased reinforcing effects silica can be treated with silane coupling agents to increase the interaction of the silica surface with the rubber thereby modifying the physical properties of the vulcanized rubber. The silane coupling agents which are most useful typically have or generate silanol groups which can react and bond to the silanol groups on the silica surface. The silane coupling agent can have mercaptan groups, polysulfide groups, or vinyl groups which can react with and crosslink to the diene based elastomers.